In recent years there is a need in view of environmentalism for reduced emissions from and improved fuel economy of internal combustion engines (hereinafter also referred to as engines) mounted in automobiles, and as an automobile that satisfies such need, a hybrid vehicle having a hybrid system mounted therein is practically utilized. In hybrid vehicles, idle stop control is adopted to improve fuel economy and reduce emissions. More specifically, if a hybrid vehicle stops at an intersection to wait at traffic lights, and a predetermined condition is also, simultaneously established, the fuel supplied to the combustion chamber of the engine is cut to stop the engine.
On the other hand, an engine mounted in a hybrid vehicle or a similar automobile is controlled in idle speed by a method as follows: a bypass that bypasses a throttle valve is provided at an intake air path of the engine and an idle speed control valve (ISCV) that adjusts an air flow rate in the bypass is provided to control the ISCV in angle through feedback to match actual idle speed to a target idle speed. Furthermore, recently, there is also adopted a method in which neither the bypass nor the ISCV is provided and instead an electronically controlled throttle valve is provided at an intake air path of an engine and adjusted in angle to control idle speed to match actual idle speed to a target idle speed.
In such idle speed control, a learning control is provided. More specifically, an amount of intake air that corresponds to an angle of the throttle valve (or ISCV) that matches actual idle speed to a target idle speed is learned as a learned ISC value (a feedback control value) which is in turn reflected in the throttle valve's angle. Hereinafter such learning control will also be referred to as ISC learning control. Furthermore in ISC learning control when a learned ISC value is an initially learned value, ISC learning control is performed in an accelerated learn mode that allows ISC learning to be increased in speed (or amount and frequency), as shown in FIG. 5, and when the learned ISC value enters the range of an extent and has thus stabilized, the accelerated learn mode is set off to switch learn modes.
Furthermore, in an engine mounted in an automobile, spark timing control is provided by a knock control system (KCS) that minimizes or prevents knocking. The spark timing control by the KCS is provided as follows: A decision is made from a signal output from a knock sensor on whether the engine knocks or not, and in accordance with the decision a spark retard is introduced with respect to a reference spark timing to combust an air fuel mixture at a reduced speed to provide a reduced, low maximum combustion pressure to minimize or prevent knocking. If a decision is made that the engine does not knock, a gradual spark advance is introduced to control and thus optimize a spark timing. Note that whenever the KCS exerts spark timing control to introduce a spark retard, the amount of the spark retard is learned as a KCS value as well. Hereinafter such learning control will also be referred to as KCS learning control. Note that an amount of spark retard is an amount that is learned to provide a spark retard when an engine knocks and provide a gradual spark advance when the engine does not knock.
Techniques relevant to such ISC learning control and KCS learning control as above are described in Japanese Patent Laying-open Nos. 4-272439 (Document 1), 5-044560 (Document 2), and 8-042434 (Document 3). Document 1 describes changing an ISC controlled target idle speed in accordance with heaviness detected by a fuel property detection sensor. Document 2 describes that when feeding fuel significantly varies gasoline in volatility, a learned air fuel ratio value is updated at an increased rate to prevent excessive overrich and overlean air fuel ratios immediately after fuel is fed. Document 3 describes that a property (octane number) of the fuel that is used is determined from a knocking control value assumed at a time of spark, and a learned knocking value, and in accordance with such decision a reference spark timing for operation at idle is set.
If in a hybrid vehicle that provides ISC learning control and KCS learning control for its engine an auxiliary battery has a terminal disconnected, a battery is exchanged, an electronic control unit (ECU) is exchanged, or the like, i.e., if electrical power fed from the auxiliary battery to the ECU is interrupted, which will also be referred to hereinafter as “battery clear”, then the learned ISC value and learned KCS value stored in memory internal to the ECU are initialized.
If after such “battery clear” the auxiliary battery is connected and the engine's operation is resumed both at idle and with a load, then ISC learning control and KCS learning control are performed without a problem and a learned ISC value and a learned KCS value are updated from their default (or initial) values. There is a case, however, in which after the auxiliary battery is connected, the engine is operated only at idle, and for a period of time it is not operated with a load. For example if a hybrid vehicle which has its engine in poor condition or is to undergo a simple inspection is passed to a car dealer for inspection, and the auxiliary battery has the terminal disconnected and again connected and thus resumes feeding electrical power, the car dealer then may only operate the engine at idle for inspection. Accordingly after the inspection before the user of the vehicle actually drives it the engine may not be operated with a load. In such a situation, while ISC learning control may proceed, KCS learning control may not, and in such a condition, the engine races, fails to satisfactorily start, provides poor fuel economy attributed to an increased amount of fuel when the engine is operated at idle.
Furthermore in an engine that provides ISC learning control and KCS learning control an amount of intake air also varies depending on a learned KCS value. Accordingly for example if regular gasoline is switched to high octane gasoline (or vise versa), and as a result the current octane number has varied and accordingly a learned KCS value has significantly varied, then a large discrepancy would be introduced between the learned ISC value as controlled and the actual amount of intake air and thus invite a poor state of operation. Furthermore, if a learned KCS value significantly varies, ISC learning control is also negatively affected, as will be described hereinafter.
Initially, as shown in FIG. 5, ISC learning control is performed from an initial state, with an auxiliary battery connected, in the accelerated learn mode. There is a case, however, in which before KCS learning control is performed a learned ISC value may stabilize and the accelerated learn mode may be set off. If the accelerated learn mode is thus set off, i.e., ISC learning is updated in a small amount, and a learned KCS value significantly varies, then ISC learning control cannot follow immediately, and a disadvantageously long period of time is required (or the learning needs to be done a disadvantageously large number of times) before an optimum, converged learned ISC value is obtained. In particular, if for increased fuel economy and reduced emissions a hybrid vehicle has its engine automatically stopped when a condition for stopping the engine is established, i.e., if idle stop is introduced in the vehicle, and for the above ground there is a long period of time before idle learning control completes, then the effect that idle stop has to enhance fuel economy is disadvantageously reduced.